2. Types of Thunderstorms 1.Airmass or Ordinary Cell Thunderstorms 2.Supercell / Severe Thunderstorms Limited wind shear Often form along shallow boundaries of converging surface winds Precipitation does not fall into the updraft Cluster of cells at various developmental stages due to cold outflow undercutting updraft

3. ORDINARY CELL THUNDERSTORMS 1.CUMULUS STAGE Sun heats the land Warm, humid air rises Condensation point is reached, producing a cumulus cloud Grows quickly (minutes) because of the release of latent heat Updrafts suspend droplets ‘Towering cumulus’ or cumulus congestus

4. 2. MATURE STAGE Droplets large enough to overcome resistance of updrafts (rain/hail) “Entrainment” Drier air is drawn in Air descends in downdraft, due to evaporative cooling and falling rain/hail Anvil head when stable layer reached (cloud follows horizontal wind) Strongest stage, with lightning and thunder

5. Mature, ordinary cell thunderstorm with anvil head

7. Aviation Risk even in Ordinary Cell (Airmass) Thunderstorms

8. Microbursts create aviation hazards

9. 3. DISSIPATING STAGE Updrafts weaken as gust front moves away from the storm Downdrafts cut off the storm’s “fuel supply” Anvil head sometimes remains afterward Ordinary cell thunderstorms may pass through all three stages in only 60 minutes

10. Review of Stages: Developing (cumulus), mature and dissipating

11. Thunderstorms Typical conditions: 1. Conditional instability 2. Trigger Mechanism (eg. front, sea-breeze front, mountains, localized zones of excess surface heating, shallow boundaries of converging surface winds)

12. Conditional Instability

13. 1. Heating within boundary layer Air trapped here due to stable layer aloft increasing heat/moisture within boundary layer (BL). 2.External trigger mechanism forces air parcels to rise to the lifted condensation level (LCL) Clouds form and temperature follows MALR 3.Parcel may reach level of free convection (LFC). Parcel accelerates under own buoyancy. Warmer than surroundings - explosive updrafts 4.Saturated parcel continues to rise until stable layer is reached

14. CAPE Convective available potential energy (J/kg)

15. CAPE (J/kg) 0 Stable <1000 Marginally Unstable 1000-2500 Moderately Unstable 2500-3000 Very Unstable >3500 Extremely Unstable

18. The Severe Storm Environment 1.High surface dew point 2.Cold air aloft (increases conditional instability) 3.Shallow, statically-stable layer capping the boundary layer 4. Strong winds aloft (aids tornado development) 5. Wind shear in low levels (allows for long-lasting storms) 6.Dry air at mid-levels (increases downdraft velocities)

22. A squall line (MCS)

23. Radar image of squall line

24. Wind shear and vertical motions in a squall line thunderstorm

25. Mesoscale convective complex (MCC)

26. Thunderstorm movement in a MCC

27. Outflow Boundaries

28. See: http://rsd.gsfc.nasa.gov/rsd/movies/preview.html

29. Supercell Thunderstorms Defined by mid-level rotation (mesocyclone) Highest vorticity near updraft core Supercells form under the following conditions: High CAPE, capping layer, cold air aloft, large wind shear Wind shear separates updraft from downdraft so it can keep developing

31. Tornado Development 1.Pre-storm conditions: Horizontal shaft of rotating air at altitude of wind shift (generally S winds near surface and W winds aloft) 2. If capping is breached and violent convection occurs, the rotating column is tilted toward the vertical

34. Tornadogenesis 1.Mesocyclone 5-20 km wide develops 2.Vortex stretching: Lower portion of mesocyclone narrows in strong updrafts 3.Wind speed increases here due to conservation of angular momentum 4.Narrow funnel develops: visible due to adiabatic cooling associated with pressure droppage

35. Wall Cloud

40. 2 hours after the Lethbridge tornado

42. Tornado producing supercell [insert fig 11-29]

43. Multiple suction vortices greatly increase damage [insert fig 11-37]

44. Global tornado frequency [insert fig 11-32]

45. [insert table 11- 2]

46. Waterspouts –Similar to tornadoes –Develop over warm waters –Smaller and weaker than tornadoes

47. Distribution of lightning strikes [insert fig 11-23]

48. Lightning Source of lightning: the cumulonimbus cloud Collisions between ice crystals and graupel/hail surrounded by supercooled water droplets cause clouds to become charged Most of the base of the cumulonimbus cloud becomes negatively charged – the rest becomes positively charged (positive electric dipole) Net transfer of positive ions from warmer object to colder object (hailstone gets negatively charged & fall toward bottom - ice crystals get + charge) Result: positive charges well aloft, negative charges near the cloud base

50. Development of cloud to ground Lightning (20% of cases) Stepped leader approaches ground Charge separation Spark surges up from ground Positive charge surges upward from ground

51. Positive strikes Particularly deadly 1.Surprise! occur outside of stormiest area 2.Tend to be stronger

52. Flashes per square kilometre per year

56. Summary of Lightning Facts Intracloud Discharges Cloud to Ground Discharges - death and destruction of property - disruption of power and communication - ignition of forest fires - Lightning is an excellent source of soil nitrogen!

57. Cloud-ground lightning 90% induced by negatively charged leaders 10% induced by positively charged leaders Sometimes, there are ground to cloud leaders Negative cloud-ground lightning Leaders branch toward the ground at about 200 km/s, with a current of 100-1000 Amperes The return stroke produces the bright flash

58. Potential difference between lower portion of negatively-charged leader and ground ~10,000,000 + V As the leader nears the ground, the electric potential breaks the threshold breakdown strength of air An upward-moving discharge is emitted from the Earth to meet with the leader

59. The return stroke lasts about 100 microseconds, and carries a charge of 30 kiloAmperes, producing the main flash The temperature along the channel heats to 30,000 + K, creating an expanding high pressure channel, producing shockwaves This results in THUNDER!